The increasing demand for gasoline and other light petroleum fractions and the increasing lack of crude oil in recent years have caused an increased interest in preparing the desired products from alternative raw materials. There has already been described, i.a. in the patent literature, a large number of processes for preparing various petroleum fractions from various fossil fuels.
According to one of these processes hydrocarbon mixtures, including high octane gasoline, are prepared directly from lower alcohols and/or corresponding ethers by the catalytic reaction over synthetic zeolite catalysts. The process and the catalysts are described, i.a., in the U.S. Pat. Nos. 3,702,886, 3,709,979, 3,832,449, 3,899,544 and 3,011,941. The alcohols and/or ethers usable as starting materials may have been prepared in separate plants. It has been found advantageous, however, to integrate the process for the preparation of hydrocarbons by the aid of zeolite catalysts with a process for preparing the said alcohols and/or ethers from a suitable raw material such as natural gas or coal. Such integrated processes have already been described and especially two of them have been the subject of interest. In one of them a synthesis gas containing hydrogen and carbon oxides is converted via methanol (MeOH) into hydrocarbons, and in the other the conversion into hydrocarbons takes place via methanol/dimethyl ether (MeOH/DME). The conversion of MeOH is essentially according to the reaction EQU CH.sub.3 OH.fwdarw.(CH.sub.2)+H.sub.2 O (5)
and that of DME according to EQU CH.sub.3 OCH.sub.3 .fwdarw.(CH.sub.2).sub.2 +H.sub.2 O (6)
In cases where it is desired to accomplish the integrated hydrocarbon synthesis at a comparatively low pressure, e.g. at a pressure of the order of magnitude of 30 bar, which occurs in industrial coal gasification plants, the degree of conversion in a process via MeOH will be comparatively low because of equilibrium conditions in the reaction: EQU CO+2H.sub.2 .revreaction.CH.sub.3 OH (1)
It will therefore be advantageous to carry out the reaction via MeOH/DME: EQU CO+2H.sub.2 .revreaction.CH.sub.3 OH (1) EQU 2CH.sub.3 OH.revreaction.CH.sub.3 OCH.sub.3 +H.sub.2 O (2)
whereby the degree of conversion is increased because of more favourable equilibrium conditions. At the same time the process exhibits only slight pressure dependency.
Such a process for conversion via MeOH/DME is described in U.S. Pat. No. 3,894,102. In the process according to that specification a mixture of carbon monoxide and hydrogen is brought into contact with a catalyst mixture consisting of a methanol synthesis catalyst and an acidic dehydration catalyst in a first step to form an intermediate having a high content of DME. The intermediate or part thereof is thereafter reacted in a second step over a zeolite catalyst to form a product containing high octane gasoline. The process according to the patent specification may be accomplished according to a number of alternative embodiments which, e.g., may be subdivided as follows:
(A) The entire amount of intermediate from the first step is conducted to conversion in the second step and recycling is not employed.
(B) One or more components of the intermediate from the first step is conducted to conversion in the second step and the remaining components are removed and/or recycled to the inlet to the first step.
Both embodiments (A) and embodiments of type (B) entail drawbacks.
The embodiment (A) gives a low degree of conversion of the synthesis gas because recycling is not employed.
Embodiments of the type (B) are uneconomic in operation and require large investments because the fractionation of the intermediate from the first step is complicated and requires cooling to a low temperature of the entire amount of intermediate and renewed heating of the part of the intermediate to be reacted further in the second step.
It has now been found that in the process according to the present invention one is not only able to avoid the drawbacks described but also able to achieve further advantages.